1. Field of the Invention
The present invention relates to a displacement sensor for use in displacement detection of an encoder, a method for detecting an optimal reference position for the displacement sensor, and a method for judging stability of the detected optimal reference position.
2. Description of the Related Art
As a general example of an encoder according to a conventional art, an optical encoder has been known. That is, this optical encoder includes: a light source section in which a light emitting section such as LED is combined with an optical device for converting a beam light from the light emitting section into a parallel light; an index scale and scale successively irradiated with the parallel light from the light source section; a photodetector disposed on a side opposite to the light source section as seen from the scale; and a signal processing circuit section for processing a signal from the photodetector to output an encoder signal. In this constitution, the index scale, scale, and photodetector are constituted of parts for detecting a position and reference position. That is, slit parts for detecting the position and reference position are disposed in the index scale and scale, additionally the photodetector also includes photo-detection parts for detecting the position and reference position, and position detection/reference position detection is possible by a combination of these.
Moreover, in objects for detecting the position such as a stage and motor, the combination of the light source section and photodetector is attached to either one of movable and fixed parts. The scale is attached to the other of the movable and fixed parts.
Moreover, general encoder output signals include: sinusoidal signals which are outputted with the displacement of the movable part of an apparatus and which are different from each other in phase by 90°; A and B phase signals which are pulse (rectangular wave) signals; and Z phase signal (signal for detecting the reference position) outputted when the reference position of the movable part is detected.
Furthermore, an absolute position of the movable part is unclear when power supply of the apparatus is turned on or position information is cleared, and in this case the reference position can be detected to obtain exact position information.
Additionally, among the encoders, in a linear encoder for linearly detecting the position, a pattern for detecting the reference position is disposed in the vicinity of one end of a linear scale in many cases. In a rotary encoder for detecting a rotary movement, the pattern for detecting the reference position is usually disposed in one place on a disc-shaped scale.
Moreover, the movable part is moved to detect the pattern for detecting the reference position of the scale, and a reference position signal is outputted in synchronization with AB phase signals if necessary.
For example, in Jpn. Pat. Appln. KOKAI Publication No. 8-261795, a mechanism for generating and detecting a signal having a period longer than that of a position detection signal is disposed in the scale, index scale, and light receiving section. That is, in addition to the displacement signal and reference position signal, an auxiliary reference position signal is generated, and is used as an index in synchronizing the reference position signal with the position detection signal.